Field of the Invention
The present invention relates generally to natural gas dehydration.
Related Art
Natural gas from underground resources is commonly mixed with other hydrocarbons, such as ethane, propane, butane and pentanes; water vapor; hydrogen sulfide; carbon dioxide; helium; nitrogen; etc. The gas is often transported through a network of pipelines that can stretch thousands of miles. The gas is usually processed to separate the various hydrocarbons and fluids to produce pipeline quality dry natural gas. The Gas Processors Association sets forth pipeline quality specifications for gas that the water content should not exceed 7 lb/million standard cubic feet (“MMSCF”). The natural gas from underground resources usually contains a large amount of water, and can be completely saturated. The water can cause problems to the pipeline, such as freezing at low temperatures, and forming hydrates with carbon dioxide and hydrocarbons that can clog equipment and pipes or cause corrosion.
While most of the water is removed from the natural gas at the wellhead by simple methods, dehydration units are often used to remove the water vapor from the gas. One method of removing water vapor utilizes a liquid desiccant dehydrator, such as a glycol dehydrator. Glycol, which has an affinity for water, is used to absorb the water vapor from the natural gas. The natural gas and glycol are brought together in a contactor. The desiccant or glycol bearing the water out of the contactor is referred to as rich or wet and becomes heavier and sinks to the bottom of the contactor where it is removed. The gas with the water vapor removed is referred to as dry gas and exits the contactor to a storage tank. Small amounts of methane and other compounds can also be found in the glycol. A flash tank can also be used to decrease the amount of methane and other compounds by reducing the pressure of the glycol allowing the methane and other hydrocarbons to vaporize or flash. The rich or wet glycol is feed to a stripper or regenerator with a column or still, an overhead condenser, and a reboiler. The stripper or regenerator vaporizes the water vapor, which has a boiling point of 212 degrees Fahrenheit while glycol has a boiling point of 400 degrees Fahrenheit. One problem with prior art strippers or regenerators is that the reboiler runs sporadically, i.e. turns on and off, such that the glycol temperature can vary by 50 degrees.
Dehydrations system also commonly use a jet-gas system which requires a large mass flow of dry jet gas to drive hot glycol circulation in the winter.
Enhancement methods to dehydration systems often involve lowering the pressure in the system to increase stripping, using a vacuum to lower the entire stripper pressure, using stripping gas, using a recoverably hydrocarbon solvent, or withdrawing partially condensed vapors from the bulk liquid in the reboiler.
In addition, cold climates require more thorough and expensive glycol dehydration. Furthermore, new environmental regulations require the removal of BTEX (benzene, toluene, ethylene and xylene) compounds from the still vents of natural gas dehydrators.
Improving the dehydration process is an ongoing endeavor.